1. Field of the Invention
The present invention relates to a low noise level internal combustion engine and, more particularly, to an internal combustion engine having a multi-cylinder which can suppress engine noise transmitted to a cylinder block from a journal portion of a crankshaft at a considerably low level.
2. Description of the Related Art
Generally, in conventional reciprocating piston engines, a journal portion of a crankshaft is rotatably supported by main bearings whose upper-half parts are carried by a cylinder block.
The lower-half parts of the main bearings are carried by bearing caps.
The cylinder block is integrally provided at its bottom portion with a skirt portion. The skirt portion is bulged out to cover the rotating crankshaft with a space between the crankshaft and the skirt portion. An oil pan for reserving an engine oil is fixed to the skirt portion.
With the recent trend of higher rotation and higher output of the engine, many methods for reducing noises and vibrations of the engine have been proposed.
Since most vibrations and noises of the engine are propagated to other portions through the cylinder block portion of the engine and the bearing portions of the crankshaft, it is most important to enhance the rigidity of these parts in order to reduce the vibrations and noises.
From this point of view, a wall thickness of the cylinder block portion is increased in thickness, or formed of a high strength material to reduce the vibrations and noises. However, this proposal gives rise to another inconvenience such that the weight of the engine itself is increased.
Schemes for reducing noises and vibrations are proposed in U.S. Pat. Nos. 4,213,440, 4,230,087, and 4,412,516.
However, all of the schemes provide only the sound-insulating member-and do not provide a method which can increase a flexural rigidity of the skirt portion.